Vacuum Web Coating

Lifetime of metallized film.

Hi sir, follow up question on the lifetime of metallized but this time on the adhesion of the metallized coating on the cpp or pet film. Some customer complains that after a year their un-used laminated opp/vmcpp film delaminate. We observed that the metallized coating completely transfers to the opp. Did the bond between the cpp and the metallized was overcome by the bond of adhesive between opp and metallized coating. Did surface contamination plays part on this after a long period of storage.

Answer

As with all adhesion failures the recommendation is to first confirm the plane of the failure. The reason for this is that you do not want to be trying to solve the wrong problem.  Assuming the failure really is at the interface it is possible that this relates to the initial adhesion and then is time and temperature dependent.  Ideally the metal is bonded to the polymer at all possible points uniformly across the whole surface.  If the surface is not suitably prepared the metal will only be intermittently bonded to the polymer. In the spaces between bonds it is possible for material to migrate into the space and it is this material that can degrade the adhesion.  This migrating material could be moisture or low molecular weight unpolymerised fragments or any additives that might have been added to the polymer.

The rule of thumb is generally that the higher the original adhesion the longer the lifetime of the metal adhesion. The lower the adhesion the easier it is for material to migrate into the space between the unbonded metal and polymer swelling the space and leading to premature delamination.  Heating the material can accelerate any degradation process as it increases the rate of migration or diffusion through the polymer of any potential contaminant.

If you have a known problem of failures after a year of storage it should be possible to plot the progressive decrease in adhesion over the year.  The force used for delamination should reduce progressively.

Thus examining the surfaces may give some indication about contamination. If you have some material that is known to fail then sending this to a surface analytical laboratory and allowing them to delaminate some material in their controlled environment and then examine each of the metal and polymer surfaces by a technique such as X-ray Photoelectron Spectroscopy should allow you to identify the chemical composition of both surfaces.  If there has been some contamination it is likely that you will have the same material present on both the metal and polymer surface and it will not be the same chemistry as the original polymer surface.

There is also another factor that may be coming occurring and that is the adhesive between the OPP and MCPP may be also aging but in this case may be slightly improving in adhesion. It depends on the adhesive type but some do not fully cure immediately but take some time and so with time can increase in adhesion.  Thus if the adhesion on one side of the metal is reducing and on the other side it is increasing the failure may switch from one side to the other over time.

I hope this gives you something to work on.

Scratches during Slitting

We are having 920 mm slitters. We are facing intermittent problems of formation of scratches in the machine direction mainly near the Heavy Edge metallization. Sometimes it continues through and through till the end of the reel and sometimes it appears and then again it vanishes. Most of the time it is appearing for higher thickness ( 7 to 10 um ) films. We have tried our best with all possible trials but could not able to find the route cause and hence not able to remove the problem permanently.

Answer

The direction of the scratches is important. If they are only in the machine direction the cause becomes limited.  Scratches are produced by a roll stopping or running slowly in comparison to the web.  If the tensions are set with too much of a difference or the winding speed of the rewind poorly set it is possible to produce intermittent scratching as the tensions hunt to some point of stability.  As you are able to wind some film well this is unlikely.  If some of the rolls are only tendency driven and the bearings wear or become dry they may become more difficult to turn and so may not precisely match the web speed.

If you look at the scratches under an optical microscope it may be possible to tell if the scratches are continuous, which would suggest that a roll has stopped completely or if the scratches are made up of a large quantity of very tiny short length micro-scratches which is more characteristic of the roll not quite matching the web speed.

Scratches can also be worse if the polymer web is dirty.  If there is debris on the surface of the web and the surface is metallized the debris is metallized too. The debris can be several microns in size so that when it comes in contact with a roller it has to lift the web up if it is to pass around the roller without moving. If the tension is high it may become difficult to do this and so the debris may start to slide along the surface of the web causing scratching.  If, when the scratches become visible, you reduce the tension and the scratches reduce or disappear this may be an indication of this being the cause of the scratches.  If this is the case then improved cleaning of the web before metallizing could also help as could cleaning all the rolls in the winding system to make sure there is no residual debris on any of the rolls.

This could also explain why it is the heavily metallized edge that is the one that shows more scratching than elsewhere.  Any part of the web that is thicker then the rest will have more tension applied to it and so as the web passes around a roller it will have a greater pressure on the thicker area of the coated web i.e. the more heavily metallized region. Thus it will be this area where it will be harder for the debris to lift the web up to allow the space for the debris to pass around the roll.   

A question about Aluminium oxidation

Would like to inquire if you have a study on vacuum metallized surfaces degradation? A case in point is what is the degradation of a layer of vacuum metallized material, if it was left uncovered, meaning to say there was no PE or LDPE layer applied over it to protect it from the elements. Also how long will the surface last and what is the typical reaction it would have. Will it breakdown into powder like substance as it turns into aluminium oxide?

Answer

Vacuum metallizing is generally achieved by using an aluminium coating.  Aluminium will oxidise very readily and so all aluminium films have an aluminium oxide surface.  The thickness of the oxide surface will depend on a number of factors such as the structure of the coating as well as the time and temperature history of the coating since the point of deposition.

Aluminium when it oxidises changes density and the lower density oxide takes up more space which puts the oxide into compression.  This oxide layer in compression helps reduce the rate at which oxygen can reach the metal and so although the first monolayer of oxide will form even whilst the aluminium is still in the vacuum system subsequent layers take increasingly longer to form.  Thus the first monolayer will form immediately, the first nanometer about 1 week, the second nanometer around 1 month whilst the third nanometer may take a year to form.

This is an idealised view of how the aluminium ages through oxidation.  The reality will be different and will also vary because of other factors.  The humidity can affect the corrosion rate particularly if the atmosphere is contaminated. Any airborne contamination will become attached to the moisture and this can form something more corrosive than simple water.

Aluminium coatings also have pinholes in the coatings and these pinholes are not simple holes pierced through a full thickness aluminium coating but are as a result of the aluminium depositing onto some debris and when the debris is moved a pinhole is produced.  The edges of the pinholes gradually thin from the full aluminium thickness to no thickness. Thus pinholes are a source of a more rapid observable corrosion as the thinner aluminium becomes more transparent more quickly than the full thickness aluminium.  This observable increase in pinholes size tends to attract attention but should be more of an incentive to reduce the number of pinholes rather than worry about the corrosion.

The structure of the aluminium can affect the corrosion rate because of the structural defects such as the grain boundaries and any voids.  These defects can be a method of wicking down moisture at an increased rate.  If the aluminium could be deposited at a single crystal the corrosion would only be possible from the front surface, however because of the crystal structure the corrosion can also take place from the crystal edges to. The surface oxide can still reduce these effects as the oxide layer builds up and the protective compressive layer is formed.  The wetting of the aluminium onto the substrate is important as too is getting high adhesion.  Good wetting and adhesion reduces any interfacial gaps where moisture can reside and corrode the aluminium from the interface.

The structure of the grown aluminium also affects the surface roughness and this can also affect the appearance of the aluminium.  If the crystal structure is coarse the surface roughness will be greater than if the crystal structure is fine. The finer structure is produced with faster deposition rates. Coarser structures are produced with lower deposition rates.  If the surface is rough then the oxide layer can start to affect the surface appearance more rapidly with the reflectance falling and the surface looking matt or even milky in colour.  Thickness also has an effect as thicker films generally have a rougher surface than thinner layers for the same deposition process.

So as you can see it is very hard to predict the lifetime of any particular coating.

It is possible to suggest some trends.  Such as higher temperature and/or higher humidity uses of metallized film are likely to show faster corrosion than for those used in lower temperature and humidity applications.

A problem of curl in metallized paper

We are facing a problem of curling in the metallized paper.

After top coating the metallized paper, the paper curls.

We have tried to give steam on the back side of the paper just before the rewind station.

At that time, it seems that the paper is not curling.

But after a couple of days, after slitting the paper if starts curling.

1.What is the best way of measuring the curl in a paper?

2.What is the sureshot way of producing curl free paper?

3.Does giving steam helps in reducing the curl in the paper?  If so, how much moisture should be added in the paper? Should we add some other material in the steam?

4.What can be the different causes of curling in the paper?

Please guide us on these issues.

Answer

A simple way of measuring curl is to cut a thin strip of material and let it rest on its edge on a flat surface and measure the radius of curvature.  This can mean cutting strips at an odd angle to get a sample that only has a curve and not also a twist in the strip. The curvature allows you to calculate the bending stress on the strip.

Curl is a measure of a difference in stress between two layers.  This is most commonly caused by the swelling and drying of one or more layers of material.

The first thing to do is to establish the sense of the stress.  If the metal surface is on the inside of the curve then the coating added on top of the metal would appear to have contracted.

If the curvature is such that the metal surface is on the outside of the curve then it is the paper that has contracted most causing the curvature.

If it is the coating on top of the metal that has contracted then this might be caused by the coating drying too rapidly so that the coating cures from the top surface first and then through the coating thickness. This solidifies the top surface as a skin and then the solvent has to be extracted from below this skin and as it is removed the lower levels want to shrink but as the top surface is fixed it puts stress into the coating.  If the drying is slower the solvent has time to migrate out from the depth of the coating with the solids settling down to the surface and this reduces the stress in the coating.

If the problem is a problem of shrinkage of the coating then adding water to the paper could be helping and is something that is worth checking out.

If you reduce the water does the problem get worse and start sooner?  If so then it might be that adding more water will swell the paper more before the coating is added so that as the coating shrinks the paper shrinks a similar amount.

If the curvature is the opposite direction with the metal surface outside the curvature it may be that the addition of moisture is the problem. As the water is dried off possibly more slowly than the coating is dried it contracts more than the coating and thus the curvature is the opposite direction. Again the test is to reduce the water addition and see what happens to the curvature.  What you are looking for is a balance to the shrinkage over the longer timescale.

Bear in mind that the drying rates for the paper and the coating may well be different, which is why you see no curl early on and curl later. What you want is no curl when the material is used later and so it may be that you will see curl when you test immediately after coating and drying but have to trust that the additional time and drying will improve the film to bring it too a neutral stress point with no curl.  So when you do your changes in moisture level you need to not only check the curl immediately but also plot any changes to the curl over time.

Where stress is concerned and curl has been a continuing problem at least one company solved the problem by coating both sides of the substrate with the same material so that the stress remained balanced. This was a drastic solution but in their case cost was not an issue and it did stop the problem. 

I hope these suggestion help.

Low adhesion on film that passes tape test

We are coating & embossing on 12 micron polyester film. After that tape test ok.
After metallization called is holography film (with plasma treatment) tape test also pass & dyne value 58+.  When we are laminate with reverse printing pet film with holography film after that found the bond strength is very poor.

Answer

The tape adhesion test does not measure adhesion it only measures lack of adhesion.  If a coating comes off with the tape test it has very bad adhesion.

If the tape brings off the coating all it proves is the adhesion is anything from just a little better than the tape adhesive all the way through to adhesion such that the failure plane will be a cohesive failure within one of the layers and not at the interface.

I would check your deposition process to see if there has been any type of pre-treatment to the polymer film before metallization. If there is no pre-treatment then using a pre-treatment could improve the adhesion. If you are using a pre-treatment then there are two possibilities. One is that the treatment is not enough to maximise the adhesion and the second is that the treatment is too much and the surface has been damaged such that you have gone past the peak adhesion to a lower level off adhesion.

If you have a pre-treatment then review how the level of treatment was optimised. If it wasn't optimised then start from a low level and increase the treatment to see where the optimum level is. This will probably require using the lamination to test the adhesion level as the tape test does not have the range of adhesion to give any meaningful answers.

Adhesion failure problem

We have 2 metallizer in our company. We have faced the metal peeling off problem. We have -20oC cooling in our chill drum and using the gas wedge also. After metallizing we measuring the temperature its 40oC . How to reduce the output roll temp? Waiting for your reply.

Answer

There are a number of aspects to your problem.

In general if you cool down the deposition drum from -20 Deg C to -30 Deg C the final temperature could be expected to reduce by a similar amount from 40 Deg C to 30 Deg C.

This reduction of the drum temperature may not be possible; it depends on the cooling capacity of the system.

A second method of improving the cooling is to increase the gas flow to the gas wedge. The heat transfer coefficient is dependent on the volume and pressure of gas trapped between the film and drum. The higher the trapped pressure the more gas collisions on both the hot film and cold deposition drum and so the higher the heat transfer coefficient.

Again this might not be possible in your system as there will always be a proportion of the gas the leaks out of the edge of the film as it passes around the deposition drum and this has to be pumped away.  IF you increase the gas into the wedge the leaking gas might take the chamber pressure higher that you would like for your metal deposition process.

A final possibility is to replace one of the standard rolls following deposition with a cooled roll.  This needs to be chosen well as it not only requires the roll to be cooled but also the film has to have sufficient wrap around the cooled roll to take benefit of the cooling. If the wrap is too short there will not be sufficient time to remove enough heat to make a significant difference to the temperature.

This final solution does require changing the winding system and requires an additional leadthrough for a further cooling liquid.

If the metal coating is flaking off this is more indicative of not having the right level of adhesion rather than it being a problem of overheating.  Adhesion can be poor because of too little or too much plasma treatment.  Have you optimised the plasma treatment on the film?

If not I would suggest that this might be more useful than reducing the temperature of the final roll alone. The roll is not very hot it is warm and so I would have thought it unlikely that reducing the temperature is going to make too much difference to the metal adhesion.  I would suspect that even after increasing the cooling the metal would still be prone to peeling off.  Check what type of plasma treatment is done, is it argon, argon/oxygen and has it been optimised for that grade of film?  If the process has not been optimised it could easily be either too little or too much and giving the poor adhesion. The treatment can be too much where the surface energy would be reading a high value but the surface damage would be enough to generate low molecular weight chain fragments that the metal would adhere to but where the low molecular weight material is no longer well bound to the polymer film.

I hope these suggestions help.

Delamination of holographic film.

We are manufacture solvent bais holography film.
After holography tape test is 95% pass but we are facing the continuous delamination problem

Answer

The tape adhesion test does not measure adhesion it only measures lack of adhesion.  If a coating comes off with the tape test it has very bad adhesion.

If the tape does not bring off the coating all it proves is the adhesion is anything from just a little better than the tape adhesive all the way through to adhesion such that the failure plane will be a cohesive failure within one of the layers and not at the interface.

I would check your deposition process to see if there has been any type of pre-treatment to the polymer film before metallization. If there is no pre-treatment then using a pre-treatment could improve the adhesion. If you are using a pre-treatment then there are two possibilities. One is that the treatment is not enough to maximise the adhesion and the second is that the treatment is too much and the surface has been damaged such that you have gone past the peak adhesion to a lower level off adhesion.

If you have a pre-treatment then review how the level of treatment was optimised. If it wasn't optimised then start from a low level and increase the treatment to see where the optimum level is. This will probably require using the lamination to test the adhesion level as the tape test does not have the range of adhesion to give any meaningful answers.

Solvents are often a problem in causing delamination but it often only happens where the adhesion is already not as good as it could be. The better the coating is bonded to the substrate the more difficult it is for the solvent to migrate into gaps in the interface.  If the adhesion is poor the solvent can migrate into the interface and any swelling will put a force on the interface encouraging delamination. Defects in the substrate surface or coating can often be traced back as the source of delamination. The defects have poor adhesion associated with them which allows in the solvent and the delamination spreads from there.

The same can be true of pinholes where after lamination the air in the pinhole can expand and the pressure of the volume change can start a delamination if the adhesion is not good enough.

Question re. Lamination bond strength.

One more question to you that is we are facing problem of less bond strength especially when the metalized film is laminated to other substrates. So to get more bond strength what we have do please advise and if it is not possible please tell us that why we cannot get good bond strength.

Answer

The bond strength after metalization can be time dependent.

It may also be dependent upon the substrate and processing.

Assuming you are referring to BOPP as per the previous questions.

Often BOPP contains additives such as slip agents that are added to the bulk polymer and which are designed to migrate from the bulk t the surface. These slip agents help make the polymer film more easily handled on winding and packaging machines by lowering the coefficient of friction.  Unfortunately this reduction in coefficient of friction generally means the surface energy is reduced and this reduces the metal adhesion. To combat this reduction in adhesion it is common to use flame, corona or atmospheric plasma treatments or vacuum plasma treatment to increase the surface energy to improve the adhesion and wetting of the aluminium.

The additives do not discriminate between surfaces but migrate to both surfaces equally. Thus when the metallized film is re-wound the freshly metallized surface will be brought into contact with the back surface of the film that has the low surface energy slip agent present. The freshly metallized surface has a very high surface energy. It is the nature of things that surfaces try to reach equilibrium at the lowest surface energy state. Hence there is a high driving force for the low surface energy slip agent to migrate from the back surface onto the freshly metallized surface. Thus reducing the surface energy and in so doing reducing the adhesion of any subsequent coating or laminate.  The longer the time and the higher the temperature between metalization and subsequent process the more material will have been transferred and the lower the surface energy and so the expectation of adhesion should also be lower.   

It may be worth reviewing you process to see if you have a consistent time between metalization and lamination. Also, if you are storing the rolls of film, review if the time, temperature and humidity are controlled.  It would also be worth checking the surface energy of the film both immediately after metalization and then over time to monitor the progression of reduction of surface energy.

Assuming the above explanation allies to your situation then what you can do to rectify the problem would be to laminate as soon after metalization as possible. In this way you can minimise the time available for any low molecular weight material to migrate across to the metal layer. This may still not be acceptable as it is also common for film to be wound in vacuum systems still warm (well above ambient temperature) and so there may still be enough time for material to migrate.  If there is still a concern over adhesion then it is always possible to treat the metal surface immediately before lamination. Using either a corona or atmospheric plasma it is possible to clean the metal surface and raise the surface energy to increase the wetting and adhesion of the laminate.

I hope this explains what might be happening to your film.

Delamination problem

In adhesive-laminated 3-ply structures of reverse-printed PET:metPET:LLDPE sealant web, the typical structure failure mode is often peeling of the metallized layer away from its base substrate, even when high adhesion metPET films are used. Some competitive & comparable Japanese and European 3-ply structures do not exhibit this weak peeling or decaling

failure mode, exhibiting outer PET film tear instead (i.e. 'destruct' bonds).

Is this adhesive technology related?

High corona treatment of the metPET?

Can you explain?

ANSWER

Getting high adhesion metallized film can be problematic, particularly as measuring the metal adhesion can be difficult to do well.  Often the only adhesion test done for metallized film is the 'tape' test which is a very poor test which only allows you to eliminate the poorest metallized coatings. 

Part of the problem is the tape test has many variables such as the age and type of tape used, the humidity when the tape was manufactured as well as when it was used, the pressure used to apply the tape and the speed and angle of pull when it is removed, etc.  Thus the test has huge error bars and cannot prove very high adhesion but can only show very poor adhesion.

Corona treatment is used to improve adhesion by increasing the surface energy which improves the wetting if the aluminium as it nucleates and the coating grows.  The corona treatment may not be a reproducible process as it too can be affected by the humidity and so the adhesion can be better in some seasons than others.  The corona treatment also declines with time. The speed of this decline is dependent upon and additives in the film and the temperature of storage.  If there are any additives, such as slip agents used to reduce the coefficient of friction to improve the handling characteristics, these will be contained within the bulk and will migrate to the surfaces as too will any low molecular weight oligomers. These will reduce the surface energy back to the starting level. The higher the temperature and the longer the time the more the benefits of the corona treatment will be lost.   Also as the front surface has a high surface energy immediately after corona treatment it will be energetically beneficial for any low surface energy, low molecular weight material to be transferred from the untreated back surface of the film to the front surface whilst the film is rolled up. Again the longer the film is stored in the roll the greater the opportunity for this material to be transferred again losing the effects of the corona treatment.

If the film also receives a plasma treatment before metallization it may be that the surface becomes over treated.

In general it is beneficial to have a plasma treatment before metallization to correct any reduction of surface energy because of newly migrated or transferred material.  However it is also possible to over-treat the polymer film surface.  It is preferable to optimise the pre-treatment process. If this includes corona as well as plasma treatment then both processes and the length of time between the two processes needs to be optimised as a total item.

If the pre-treatment is gradually increased it will be seen that the surface energy increases up to a maximum and this then plateaus at the high level. If, however, we also plot the adhesion we can see that it initially follows the same path and increases with increasing treatment. However once the maximum is reached instead of remaining high at the plateau level the adhesion immediately starts to decline with any further increase of power or treatment time.

What is happening is that the treatment that causes scission (breaking) of the polymer chains to produce new bonding sites which are often occupied with oxygen which can bind better to the aluminium.  This scission process reaches an optimum in binding sites but any further treatment continues to break chains and this results in ever shorter chain fragments. This finally results in a carbonised layer that is a very weak boundary layer and, although the aluminium may be bonded to it, the adhesion to the polymer bulk is poor because of too many short polymer chain fragments.  The chemical composition stops changing and so the surface energy remains constant at the high level but the adhesion falls away.

Thus for your laminate I would start by reviewing the whole process starting with the polymer film, checking to see if there are any additives included to improve the web handling. I would then go on to check the consistency of the corona treatment, the storage time and conditions of the film following any corona treatment. I would also check if there has been any plasma treatment as well as corona treatment and check to see if the process has been optimised for the film.  (Sometimes the conditions have been set using a different film and it is assumed the same treatment can be applied to other films - and this may not be the case).

I hope this explains what might have been happening and possibly gives some way forward to sorting out the problem.

Question re. lamination problems.

We are doing water-based lamination with metallized BOPP film we also do it with metallized CPP film. However when we do adhesive lamination with BOPP we find problem of corrosion i.e. water attacks metallization layer. We have a doubt that the composition of metallized layer being coated on CPP is different than that of BOPP as they are from two different suppliers. Could you please guide us that what could be there in the metallized layer that is so hydrophilic that it does not allow moisture to evaporate?

Answer.

Cast PP and Biaxially Oriented PP differ where the PP is a mixture of amorphous and crystalline material. The orientation process can result in some alignment within the polymer of the crystalline material. This alignment changes the performance of the polymer is all sorts of ways including tensile performance and also permeation performance.  It is likely that the moisture barrier of the BOPP is better than that of CPP.  This might be a contributing factor.

What you do not mention is what the thickness is of each of the materials and this can also be a factor.  The barrier performance of any material is thickness dependent. Thus, if the CPP and BOPP are of different thickness then this too could affect the performance.

Similarly if the metallization thickness is different in the two samples then this too could result in a differing barrier performance.  Keep in mind that if the materials are metallized by different suppliers there may also be other differences in the metallized layer.  The faster the rate of aluminium deposition the smaller will be the crystal size of the aluminium coating. If the metallization is done at different pressures this too will result in differences within the coating. Higher pressure will result in a less dense coating and lower permeability for the same thickness.  Measuring the coating thickness and also the Optical Density (OD) of each coating will give an indication of how similar the two metallization processes are.  If the thickness were the same but the OD different, or the OD the same but the thickness different it would indicate there are differences in the metallization process. If both thickness and OD are the same, or very similar, it would indicate the metallization processes are also similar.  Another difference could also be in the quality of the coating as described by the number of pinholes per unit area.  The higher the level of pinholes the greater the permeability of the coating as well as the greater the number of starting points for any corrosion.  The surface roughness of the substrate can also affect the nucleation and growth of the aluminium and hence the barrier performance. The higher the surface roughness the worse the barrier performance is likely to be.  Other factors that could affect the corrosion of the metallized film are the temperature and humidity conditions seen by the roll of material.  If the metallized film is re-wound hot, as in greater than 30 deg C, in the metallizer it is likely that the aluminium coating will grow a thicker oxide more rapidly than if the temperature at the re-wind is closer to ambient, generally less than 30 deg C.  After metallization if the rolls are stored in a high humidity atmosphere they will hydrate the aluminised film and lead to more rapid oxidation than if stored in a low humidity atmosphere. Adhesion is another possible factor that could have an effect. It would be expected that the system with the lower adhesion to suffer from higher permeation and more likely to have a greater propensity to corrosion.   I would normally expect the metal to CPP adhesion to be lower than that of metal to BOPP.   

I would suggest that measuring the barrier performance, OD, metal thickness and adhesion of each of the metallized materials before lamination would be a useful comparison. 

I would expect that the barrier performance of the BOPP would be higher than that of the CPP thus making the removal of the water from the adhesive slower than for the CPP.

I hope that this gives you some points that you find useful in helping solve this problem.